This study focuses on the development of children's reasoning about disease and treatment processes. Because children may fail to understand the abstractions related to disease and treatment, their participation in research has always presented special challenges. The role of anxiety, perception of control over illness, and stage of cognitive development were examined as possible influences on reasoning about illness. Standard measures of cognitive functioning, interview measures designed to assess reasoning about one's own illness and illness in general, and a measure of anxiety were administered to the participants. These measures permit exploration of developmental differences in comprehension of one's medical situation, and allow examination of the effects of anxiety and perception of control over illness on comprehension at different age levels. Participants are child, adolescent, and adult inpatients and a comparison group of healthy participants. Findings show that cognitive as well as emotional factors are related to child and adolescent reasoning about illness and treatment. Older and more cognitively mature children had a higher stage of reasoning about their own illness and illness in general. Anxiety level was not related directly to stage of reasoning about illness. However, children who were higher on anxiety level were lower on perception of control over illness. Promoting a sense of control over illness may help children cope with the stress that accompanies major illnesses.